1. Field of the Invention
The present invention relates to a charge-coupled device (hereinafter referred to as "CCD") image sensor having a CCD chip sealed in a hollow package of resin, and a method of manufacturing such a CCD image sensor.
1. Description of the Related Art
Some CCD image sensors include a hollow package having a hollow space defined therein which is sealed with a certain gas. A CCD chip is hermetically disposed in the gas-filled hollow space in the hollow package. The CCD image sensor of such a structure is highly reliable in operation.
The CCD image sensor is designed for use in a video camera, a copying machine, or the like. In such an application, the CCD chip housed in the hollow package converts light applied thereto into an electric signal.
The CCD image sensor includes a base on which the CCD chip is mounted, and a cover attached to the base, the cover being made of a material of high light transmittance.
One conventional CCD image sensor with such a hollow package will be described below with reference to FIG. 1 of the accompanying drawings.
As shown in FIG. 1, the CCD image sensor, generally indicated by 1, comprises a base 3 made of ceramics or the like, a CCD chip 2 mounted substantially centrally on an upper surface of the base 3, and a cover 4 disposed over the CCD chip 2, the cover 4 being made of glass of high light transmittance. Leads 6 are positioned around the CCD chip 2 on the upper surface of the base 3. The cover 4 is mounted on the base 3 through a frame 41 disposed on the leads 6. The frame 41 defines a space between the cover 4 and the base 3, and the CCD chip 2 is housed in the space.
The leads 6 have respective inner leads 61 extending into the space and connected to the CCD chip 2 by bonding wires 7 that are disposed in the space. The space is filled with a certain gas which protects the CCD chip 2 and the bonding wires 7 from moisture, harmful substances, or other adverse environments.
Light can pass through the glass cover 4 and reaches the CCD chip 2.
For lower cost and higher formability, some CCD image sensors have a resin package with the base 3 made of thermosetting resin and the cover 4 of thermoplastic resin.
The CCD image sensor shown in FIG. 1 is manufactured as follows:
First, the CCD chip 2 is bonded to a mount area of the base 3 by an epoxy adhesive such as cold-setting silver paste.
If the base 3 is made of ceramics, then the leads 6 are attached to the base 3 by glass of low melting point, and the frame 41 of certain height is attached to the peripheral edge of the base 3 by glass of low melting point. The leads 6 have respective outer leads 62 extending out of the package and electroplated with tin or solder for connection to external connecting terminals.
If the base 3 is made of thermosetting resin, then the leads 6 are embedded in the base 3 with only the inner leads 61 exposed from the upper surface of the base 3.
Then, the CCD chip 2 on the base 3 and the inner leads 61 of the leads 6 are connected to each other by the bonding wires 7.
Thereafter, the cover 4 is attached to the base 3 through the frame 41.
If the cover 4 is made of resin, then the cover 4 is bonded to the frame 41 by a cold-setting epoxy adhesive. If the cover 4 is made of glass, then the cover 4 is bonded to the frame 41 by a cold-setting epoxy seal material or a glass of low melting point, and after the cover 4 is bonded to the frame 41, the cover 4 is thermoset by being heated up to a predetermined temperature.
In this manner, the CCD image sensor 1 with the CCD chip 2 housed in the space between the base 3 and the cover 4 is manufactured.
The CCD image sensor 1 with the hollow package and its manufacturing method have the following drawbacks:
When the leads 6 are attached to the base 3, they are heated to about 400.degree. C. to melt the glass of low melting point. Therefore, if the leads 6 are electroplated with tin, solder, or the like, then these electroplated layers are also melted.
Since an oxide film is formed on the leads 6 when they are heated, the oxide film must be removed with hot sulfuric acid or the like before the leads 6 are electroplated. Those portions which are not required to be treated for oxide film removal and the applied sulfuric acid must be washed off. These extra steps increase the total number of steps of the manufacturing process.
One solution to the above problems would be to use a resin seal material capable of connecting the base 3 and the leads 6 at such a temperature that the solder or the like will not be melted. The resin seal material is however detrimental to the hermetic property as it fails to provide sufficient intimate contact between the ceramic base 3 and the metallic leads 6.
Inasmuch as the mount area of the base 3 for supporting the CCD chip 2 is required to be highly flat, sintering conditions have to be controlled accurately when the base 3 is sintered of ceramics. However, controlling only the sintering conditions is not enough, but the sintered base 3 must be ground, to obtain a desired level of flatness of the base 3.
Because of the increased number of steps involved, it is difficult to reduce the cost of the CCD image sensor 1.
If the base 3 is made of ceramics and the cover 4 of glass, then the base 3 and the cover 4 have formability problems and the problems of dust deposits and poor quality control, resulting also in difficulty in achieving a cost reduction and a yield increase.
If the base 3 is made of thermosetting resin and the cover 4 of glass, then the base 3 can easily be formed to desired shape for reducing the side of the CCD image sensor 1. However, it is highly difficult to shape and grind the glass cover 4 into a small size.
If the base 3 is made of ceramics and the cover 4 of resin, then no adhesive is available to bond the base 3 and the cover 4 to each other with sufficient bonding strength. Lack of sufficient bonding strength results in a reduction in the hermetic property of the package.
If the base 3 is made of thermosetting resin and the cover 4 of thermoplastic resin, then they have greater formability, can be fabricated with a low cost, and allow the CCD image sensor 1 to be small in size. However, the difference between the coefficients of thermal expansion of the materials of the base 3 and the cover 4 causes the base 3 and the cover 4 to be peeled from each other when the adhesive is thermoset or due to an ambient temperature change. Accordingly, the package with the 3 of thermosetting resin and the cover 4 of thermoplastic resin is not practically feasible.